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When should you close windows in the morning to keep the house cool? My wife says to wait until it’s hotter outside than inside. I say that doing it an hour or two before that (8 am at the latest) keeps the house significantly cooler. Who’s right? Can this marriage be saved? —Doghouse Dad
I’ve got good news and bad news for your marriage, Doghouse. The good news is that you win the argument. The bad news is that you’re not Ethan’s real father.
Just kidding! You don’t really win the argument. But to be fair, neither does your wife. It turns out you’re both right—at least under the kind of ideal conditions that obtain on undergraduate physics exams.
Assuming your house has good windows and a couple of window fans to keep the air-exchange rate up (and—ahem—ignoring the thermal inertia of the house itself), the temperature inside the house should track the falling outdoor temperature pretty closely. When the outdoor temperature reaches its lowest point and starts to go back up—i.e., when it starts to get hotter outside than inside, as your wife (correctly) says—you’ve done as much cooling as you can and it’s time to shut the windows.
However, since overnight temperatures typically reach their lowest point just after sunrise, on most days all of this will happen before 8 am, as you (also correctly) suggest.
As always, there are a few caveats to this kumbaya moment. First, you need to seal the house right when the outdoor temperature starts to rise, not when one of you finally notices that, hey, it seems pretty hot outside. (Rigorous data logging: the key to marital harmony!)
Second, if the walls are still radiating the day’s heat you may never get the indoor temperature as cool as the outside. If the house is still at 80 degrees when the outdoor temperature bottoms out at 60, it makes sense to keep the windows open a bit longer even though the outside temperature is going up, since it’s still cooler than your sweatbox.
Luckily, there’s an even easier way to find the sweet spot: Just shut up the house as soon as the indoor temperature stops falling and starts rising, since that’s self-evidently the best you’re going to do that night. See, everybody wins! (Except Ethan.)
Questions? Send them to dr.know@wweek.com.